Im having the same issue on my unsupported iMac running Mojave. I keep getting like a video card issue, where the screen flutters, restarts on its own, then gets stuck in the boot screen. The Apple logo will show, the status bar fills halfway, then disappears. It worked after a few restarts, but last night, it restarted on its own, and did the logo/status bar thing again. I think I just want to go back to High Sierra, which was working fine on my iMac. The problem I have with doing this is that since iTunes and Photos were last saved on Mojave, I cannot load those files in High Sierra. Is there any way around that? Or am I stuck with having to re-install Mojave every few weeks? And BTW...I cannot re-install without losing my data, there just doesnt seem to be that option anywhere.